The invention pertains to metal-cutting shears.
When using metal-cutting shears to cut sheet metal, one tries usually to utilize a large part of the length of the jaws for each cut, before opening the shears to start a new cut. The force on the jaws is very local occurring just where they cross one another, i.e., the point where the metal sheet is being parted. This is in contrast with secateurs or cable cutters where the force is distributed over a greater length of the jaw, corresponding to the diameter of the branch or cable. In sheet metal cutting, the force depends on the angle between the jaw edges, i.e., a large force when the angle is small, and a smaller force when the angle is larger. In simple metal-cutting shears, the jaws are straight, and the angle between them is reduced when the jaws get closed. For such shears the force increases successively during the cut, and at the same time the force and the crossing point are moved forward to progressively greater distances from the joint, which increases the leverage. The operator must then press the handles together with a sharply increasing force, and the full length of the jaws can not be used for cutting of thick metal, which must then be cut with very many small cutting motions, all of them with the handles far apart, which is uncomfortable and tiring for the hand, and which will produce jagged edges of the metal sheet.
It is earlier known from McGary et al. U.S. Pat. No. 2,449,561 to make the upper jaw curved to make the crossing angle and the jaw force less variable. German Patent No. 42 11 591 shows both jaws slightly curved, for the same purpose, and also shows how the leverage can be made variable by not attaching the lower jaw to the upper handle, but rather letting it be moved by a cam mechanism including a roller. This means that the lower handle griped by the user's fingers will be almost immobile when cutting, while the upper handle gripped by the user's thumb and wrist will move. This causes exhaustion of the hand. The leverage is not varied in such a way that the gripping force of the fingers is fully constant. Producing variable leverage by a sliding cam action is also known from secateurs or loppers through Kishimoto U.S. Pat. No. 5,058,277, but the gripping force will decrease markedly when the handles are closed, and that design is suited for cutting branches or cables, but not sheet metal.
Shirk U.S. Pat. No. 1,533,039 shows shears where no effort has been made to keep the cutting force at the jaws constant, and where the jaws are not shaped to let both handles be held at the same side of the sheet material. Sargent U.S. Pat. No. 4,502,222 shows metal shears where both handles are on the same side of the sheet material, but no effort has been made to keep the cutting force constant, and the leverage is obtained with a four joint design. Curved jaw edges are known from Juras U.S. Pat. No. 3,572,192, but they are not shaped to ensure a constant cutting force.